This invention pertains to X-ray image subtraction methods and to apparatus for performing such methods.
Digital X-ray image subtraction systems are now being used to visualize the blood vessels in the body. The subtraction procedure involves making an X-ray image, called a mask image, of a region of interest in the body. The mask image is digitized and the digital data representative of picture elements in the mask image are placed in a digital frame memory. At some time, usually just before the mask image is obtained, an X-ray contrast medium such as an iodinated compound is injected intravenously. When the contrast medium reaches the vessels in the region of interest, a series of X-ray images are made and they are converted to digital data. The mask or pre-contrast image data are then subtracted from the post-contrast image data to cancel or subtract out all soft tissue and bone structure common to both images to thereby enhance visualization of the blood vessels that contain the contrast medium. The method is commonly called temporal subtraction imaging because of the substantial time lapse between the pre-contrast and post-contrast images. One type of apparatus for performing temporal subtraction methods is described in U.S. Pat. No. 4,204,225.
One of the problems with temporal subtraction techniques is that there may be a substantial loss of registration between the mask and post-contrast images due, primarily, to movement of soft tissue. Movement of soft tissue or anything else between the two stored images will result in blurring or artifacts in the subtracted or difference image and will distort or obliterate the desired image of the blood vessels containing the contrast medium.
With temporal subtraction it is often possible to achieve good cancellation or subtraction of bone, which usually does not move involuntarily, but some atifacts or misregistrations may result from involuntary tissue motion such as that due to swallowing, breathing, peristalsis and blood vessel expansion and contraction.
Another image subtraction technique is characterized as energy subtraction. Energy subtraction is based on the fact that X-ray attenuation by a body or any material is an X-ray energy dependent phenomenon and that the energy dependence is different for materials having different atomic number averages. In the energy subtraction technique, an X-ray image of a region of interest in the body is obtained with a nominally low kilovoltage (kV) applied to the X-ray tube so the beam projected through the body has an energy spectral distribution within a band having low average energy. In digital fluorography, an X-ray image intensifier tube is used to obtain the image and it is viewed with a video camera whose signals are digitized and stored as an image frame. After the relatively low energy image is obtained, another image is obtained with a comparatively higher kV applied to the X-ray tube and a resulting higher average energy spectral band. For ordinary tissue studies the two images may be made in the absence of any contrast medium. For arteriographic studies, the two images are obtained when there is an X-ray contrast medium such as an iodinated compound present in the blood vessels.
In any case, the high average energy image picture element (pixel) data are subtracted from the low average image data and a difference image remains. Prior to subtraction, the data are usually variously weighted or scaled to bring about cancellation of soft tissue. The data could be scaled to reduce bone, too. However, it is not possible to remove or cancel bony structures without also removing most of the iodinated contrast medium which is really what one is trying to visualize in arteriographic studies.
There are also brightness non-uniformities in the subtracted or difference image due to several effects when the data are acquired using an image intensifier. Veiling glare, which is like haze, results from light diffusing or feeding back from areas of the input fluorescent screen of the intensifier to other areas. The fact that rays of a broad X-ray beam are scattered by body tissue in an energy dependent manner between ray paths also causes loss of image contrast. Differential detection of X-rays at various energies in the input phosphor of the image intensifier leads to additional brightness non-uniformities. None of these phenomena can be completely nullified by energy subtraction alone.
An improved hybrid subtraction method, using low and high average X-ray energy spectral bands to make exposures, has been proposed by W. R. Brody in pending patent application Ser. No. 260,694, filed May 5, 1981 now U.S. Pat No. 4,445,226. The hybrid subtraction method uses a combination of energy and temporal subtraction techniques. In hybrid subtraction, X-ray images are obtained at two different average X-ray energies, that is, with two different kilovoltages applied to the tube and the images are combined in a manner to suppress signals due to soft tissue in a heterogeneous object such as the body.
At this juncture it should be noted that the X-ray beams having low and high average energies or energy spectral bands can be obtained in various ways. One way is by applying a constant kilovoltage (kV) to the X-ray tube and interposing two different filters alternatingly in the beam. One filter is for softening the X-ray beam, that is, for removing high energy spectra above a low energy average energy band. Typically, a desired low energy spectral band is determined and a filter is chosen that has relatively low attenuation at X-ray energies below its k-edge and has high attenuation for energies above the k-edge to thereby remove such high energy spectra. A filter made of a rare earth element such as cerium or erbium are examples. The other filter is for hardening the high energy beam and would be composed of a material that attenuates or absorbs the low energy band intensely. Thus, the high energy spectra filter can be aluminum, copper or brass, as examples.
Another way to generate low and high average energy X-ray beams is to switch the X-ray tube applied voltages between low and high levels. Still another way is to switch the X-ray tube applied voltage and switch filters correspondingly. This is the preferred way.
In hybrid subtraction a mask image is obtained first by projecting a low average energy X-ray beam (hereafter called low energy beam or low energy spectral band) through the body followed by a higher average energy X-ray beam (hereafter called high energy beam or high energy spectral band) when the intravenously injected X-ray contrast medium has not yet entered the blood vessels in the anatomical region of interest. The images, consisting primarily of bone and soft tissue acquired at the two energies, are scaled or weighted, using appropriate constants, and then subtracted to produce a mask image in which signals due to soft tissue variations are suppressed and bony structures remain. The data for a pair of high and low energy X-ray images are next obtained when the intravenously injected iodinated compound or other X-ray contrast medium reaches the vessels in the region of interest. The data for this pair of images are acted upon by the same constant weighting factors that were used with the first pair of images and one image in this pair is subtracted from the other such that the resulting post-contrast image contains data representative of bone structures plus vessels containing contrast medium. The final step in hybrid subtraction is to subtract the dual energy post-contrast image from the dual energy pre-contrast mask image to thereby suppress or cancel the bone structures and isolate the contrast medium containing vessels. A major advantage of the hybrid subtraction technique over temporal subtraction alone is the reduced sensitivity to soft tissue motion artifacts because the soft tissue is suppressed or cancelled in both dual energy images.
Hybrid subtraction is a good technique for eliminating anything that may have moved during the time between obtaining the mask image and post-contrast image or images. However, if there is no movement during ordinary temporal subtraction, wherein the post-contrast image is simply subtracted from the pre-contrast mask image, then temporal subtraction images can be used because they generally have a better signal-to-noise ratio (SNR) than hybrid subtraction images. A higher SNR results in displayed images that have better contrast at a given noise level.
Scattering of the X-ray beam by the body is also considered. Scatter in an image depends on X-ray beam energy, beam path length and density of the object being penetrated. In the hybrid subtraction technique the scattering that results from use of a broad cross section X-ray beam is of little consequence since scatter is essentially the same for each energy subtracted pair of images. Hence, scatter effects on image brightness non-uniformities are subtracted out when the pairs are subtracted.
To recapitulate, hybrid digital fluorography techniques provide the merits of soft tissue motion insensitivity, effective bone cancellation, and elimination, to the first order, of scatter and other nonlinear effects in the X-ray image intensifier and the video camera.